The present invention provides an inlet air pre-cooler arrangement for gas turbines and other air breathing apparatus, but will be particularly discussed for gas turbine. More specifically, a pre-cooling system with a plurality of electable alternative modes is operable to reduce, below ambient air temperature, the temperature of the inlet air charged to a gas turbine; reduce or control the humidity in the inlet air to the compressor of a gas turbine; and, increase the density of the air communicated to the gas turbine compressor.
Gas turbines, which broadly include an air inlet, a compressor, a combustion chamber, turbines and an exhaust, compress incoming air flow for mixing with fuel prior to ignition of the air-fuel mixture in the combustion chambers for generation of high-temperature gasses to drive the turbine. Gas turbines are utilized for the generation of mechanical power for vehicles, such as aircraft, and are also coupled to an electrical generator in the electrical power-generating industry for the generation of electrical power, especially during peak-load periods. Gas turbine usage for electrical power generation, or a gas-turbine generator, is particularly prevalent during the summer months to supplement normal power (e.g., steam or hydroelectric) generating plants for peak power demand during very hot days when air conditioning demands frequently cause an increased power demand. The gas turbine generators are also used for base-load systems in smaller utilities, and for co-generation systems. However, gas-turbine-generator KW output rating or thermal efficiency is inversely proportional to the inlet air temperature to the gas turbine-generator. That is, the gas turbine-generator is less efficient with inlet air at elevated temperatures, such as 95 degrees Fahrenheit (35.degree. C.), than it is with air at 20 degrees Fahrenheit (-7.degree. C.), which fact has been known in the turbine industry for many years.
Various apparatus and methods have been utilized to reduce the temperature of inlet air to a gas-turbine-generator to minimize the impact or disadvantage on gas turbine generator output. However, the increased demand for electrical generator power output is frequently required at peak-use periods, such as hot summer days, during maximum power usage for cooling, which unfortunately is usually the time period with the highest ambient air temperature. Thus, the increased electric-power and the economic cost to reduce the temperature of the inlet air to the gas-turbine compressor is frequently unjustified, as the added power cost for inlet air-temperature reduction may be greater than any potential gains in gas-turbine power output. Therefore, the power-generating industry continues to seek methods and apparatus with minimal associated costs to lower inlet air temperature for gas turbines.
One frequently utilized air cooling apparatus for temperature reduction of gas-turbine inlet air is an in-line evaporative cooler ahead of the gas-turbine inlet, which acts as a direct evaporative cooling (DEC) apparatus. However, the temperature reduction from this DEC apparatus may only be approximately 85% of the dry-bulb/wet-bulb temperature differential. More importantly, the DEC apparatus cannot produce any significant benefit unless ambient relative humidity is well below 75%, and preferably between about 20 to 60%. As an example, ambient air with a dry-bulb temperature of about 95 degrees Fahrenheit (35.degree. C.) and wet bulb temperature of about 78 degrees Fahrenheit (25.degree. C.) may only be depressed to a dry bulb temp of about 80.5 degrees Fahrenheit (27.degree. C.). In addition, the relative humidity of this reduced-temperature air may be in excess of 90% (percent) or, in fact, may be, during sudden weather changes, water-saturated with entrained water particles, which particles may impinge and abrade or otherwise damage the turbine blades.
Although chilled or cooled gas turbine inlet air is preferred and, as noted above, aids in an increase of the gas turbine-generator capacity, the selection of a specific chilled air temperature impacts upon the output capacity of the gas turbine-generator. The temperature of the turbine compressor inlet air must be above 32 degrees Fahrenheit (0.degree. C.) to prevent ice buildup on the compressor blades, as the chilled inlet air may be at 100% relative humidity or have entrained moisture carry-over from the air-chilling process. Further, the rapid increase in air velocity in the compressor inlet results in a static pressure drop in the air of as much as 4 inches of water, which may result in a further temperature drop and moisture condensation. Thus, it may be desirable and more advantageous to have the chilled inlet air at about 45 degrees Fahrenheit (7.degree. C.) and about 85% relative humidity, which more readily accommodates variations in the air temperature and humidity while preserving the improved operational integrity of the gas turbine with the chilled air. The relative options and benefits of inlet air chilling for gas turbine utilization are discussed in the article "Options in Gas Turbine Power Augmentation Using Inlet Air Chilling", which paper was presented at the 1990 Gas Turbine and Aeroengine Congress and Exposition in Brussels, Belgium.
A cooling tower is a familiar structure associated with cooling apparatus, which cooling tower is generally a fluid (e.g. water) recirculating arrangement acting to reject heat to the atmosphere. The cooling tower usually has a heat exchange apparatus in its fluid circuit with the fluid recirculated through the heat exchanger by a pump for return to the cooling tower and gravity feeding over a heat-exchange media. The recirculating cooling tower and heat exchanger arrangement adds both heat and water vapor to the air transferred therethrough and generally requires a makeup water system as a great deal of cooling fluid will evaporate.
In a direct evaporative cooling system, which is an air cooling and humidification device circulating air over direct heat exchange air-to-water media such as an air washer, cooling and humidification generally occur when air is passed through a continuously recirculating cold water spray. This is a constant enthalpy process, as any evaporation requires heat to be withdrawn from the air (i.e., a temperature drop), and the recirculating water temperature is concomitantly reduced to the wet bulb temperature of the incoming air. After a period of time, except for slight pump-energy heating, the recirculating water attains approximately the air wet-bulb temperature through a purely evaporative means. The evaporative cooler does not utilize a heat exchanger as opposed to a cooling tower, and discharges air which is reduced in dry-bulb temperature and is essentially moisture saturated, that is it typically has a relative humidity above 90%. However, moisture in the cooled air transferred to another apparatus from the evaporative cooler should be kept from freezing when the air temperature drops and, therefore, the entering wet-bulb temperature should be kept well above 32 degrees Fahrenheit (0.degree. C.), such as 40 degrees Fahrenheit (4.5.degree. C.) minimum. Even though evaporative coolers cannot produce cold air exhaust on warm days, the air discharged from the direct evaporative cooling system will be cold, on a cold ambient day, and it may be necessary to reheat the air before sending the moisture saturated air to a gas turbine inlet, which reheating controls the humidity and allows further evaporation of any water droplets in order to inhibit moisture freezing on the downstream apparatus.
Reducing the temperature of the cooling fluid in the DEC apparatus below the entering-air wet-bulb temperature, such as by ice-water in a separate structure, can further decrease the discharged-air temperature below the ambient-air, wet-bulb temperature. When a DEC discharge water temperature falls below the entering air dew-point, this results in both an air temperature decrease below the wet-bulb temperature and dehumidification. The final temperature of discharged water will depend upon external heat removal and water quantity transferred through the air washer, but when the air dry-bulb temperature is depressed below ambient dew-point conditions, some moisture will condense from the air. Thus, depression of the coolant fluid (usually water) temperature in a direct evaporative cooler to a temperature well below the dew-point with a constant water volume flow will can depress below the dew-point both the wet-bulb and dry-bulb temperatures of the air flowing through the DEC apparatus. However, ambient air parameters such as wind velocity, temperature and humidity can fluctuate rapidly as weather patterns change, which can affect the heat transfer characteristics of a direct contact chiller (i.e., DEC-ice water and heat-removal coupled apparatus) causing excessive chilling of the discharge air, perhaps to 35 degrees Fahrenheit (2.degree. C.) or lower, which could lead to freezing deposits at the lower pressure region of a turbine inlet cone, for example. Thus, auxiliary equipment may be required to provide discharge air to a gas-turbine inlet at an assured-minimum controlled temperature and assured-minimum relative humidity.
Although it is known that chilling the inlet air for use in a gas turbine will enhance the efficiency and operation of the gas turbine, the chilled inlet air has to be provided controllably, efficiently, economically and without adding unwanted auxiliary power consumption during peak load operations. The preferred turbine inlet air "quality" is dependent upon the differential between the ambient air wet and dry bulb temperatures, as well as the desired inlet air relative humidity, the barometer pressure and the overall change in air density. All of these parameters reflect the character of the reduced temperature air and impact on the operation of the gas turbine. Therefore, it is necessary to consider all of these parameters when providing a mass flow rate of air at a reduced temperature to a gas turbine.
The precise characterization or desired air temperature and/or humidity of air conditions are noted in a psychrometric chart, which provides semi-empirical relations giving the air thermo-physical properties in terms of the barometer and psychrometer readings. The psychrometer is an instrument for measurement of the wet and dry-bulb temperatures of air. Psychrometric charts are nomograms constructed to provide convenient determinations of the properties of air-water vapor mixtures, such as humidity, dew-point, enthalpy, specific volume and water-vapor pressure as functions of barometric pressure and temperatures obtained with a psychrometer. Thus, a design inlet air temperature of 45 degrees Fahrenheit (7.degree. C.) and 85% relative humidity for a gas turbine in the example provides a reasonable operating temperature at an acceptable relative humidity with minimal concern for potential ice buildup on turbine blades while accommodating unexpected weather variations. Control of the inlet air mixture is not accommodated by use of a direct evaporative apparatus or cooling tower, as the air communicated through the tower may be reduced in temperature to approximately the wet-bulb temperature of the ambient air but it may also become water saturated. This latter condition can prevent entrained water droplets in the air flow from further evaporation, which may damage the turbine blades. Thus, it is desirable to control the humidity of the turbine inlet air to minimize the potential for transfer of entrained water droplets from the DEC to the turbine inlet. Air with a maximum of 85% relative humidity allows for further evaporation of water droplets in the air path between the DEC exit and turbine inlet.
In an article, "Advances in Technology With potential for Air Conditioning and Refrigeration" by Raymond Cohen, an alternative gas turbine air enhancement arrangement is provided by a system, which uses finned-tube closed-circuit cooling coils cooled by a glycol/water solution from an off-peak ice freezing system, using the same fluid for off-peak freezing of ice. Air is communicated past these cooling coils to reduce its temperature from a nominal reference temperature of 90 degrees Fahrenheit (32.degree. C.) to approximately 60 degrees Fahrenheit (16.degree. C.) for injection or communication to a gas turbine coupled to a generator for producing electric power. An off-peak-operable ice chilling system manufactures and stores ice during electrical power off-peak hours. The stored ice is utilized to reduce the cooling fluid temperature in the cooling coils during turbine usage for reducing the inlet air temperature communicated to the gas turbine. However, the system has no provision for control of the relative humidity, requires a high external static loss type of fin-coil heat exchange, and terminal temperature difference associated therewith, and the reduction in air temperature is limited by the single stage operation of the system without flexibility for other operating modes.
U.S. Pat. No. 4,137,058 to Schlom et al provides an indirect evaporative heat exchanger with walls having wet and dry sides for cooling a gas for a turbine compressor. The heat exchanger provides a cool dry air stream and a cool moist air stream on either side of these walls for communication to a power turbine compressor inlet and intercooler. In alternative embodiments, indirect evaporative cooling units are connected in series to provide cool dry air from the first unit as an inlet air to both the wet and dry sides of the second indirect evaporative cooler. The resultant very cool moist air from the second indirect evaporative cooler is combined with the cool moist air from the first indirect evaporative unit and is utilized for a two-stage gas compression system intercooler. The resultant cool dry air is used as an inlet air to the gas turbine air compressor. In a third embodiment, the dry cool air stream and a moist cool air stream emitted from the indirect evaporative cooler are combined and transferred for use as an inlet air stream to the turbine air compressor, however, no cool air is provided to the intercooler in this disclosed embodiment. In a final embodiment, the dry cool air stream from the indirect evaporative cooler is used as an inlet air stream for the wet side of the indirect evaporative cooler and the resultant, supposedly very cool moist air is then utilized as an inlet air charge for the turbine air compressor.
The known air-cooling apparatus, which include mechanical chillers, evaporative air coolers and absorption chillers, may provide a cool, or cooler than ambient, inlet air temperature to a gas turbine to enhance its efficiency and operating performance. However, no consideration has been given to control of entrained moisture droplet elimination, air density, relative humidity at a specific temperature in the inlet air volume, or off-peak thermal storage ice manufacture and harvest, which ice provides on-peak operations without the electrical energy demand of refrigeration compressors on-line. Further, they are a much greater first cost, are less economically operable and frequently put an added electrical burden on gas turbines coupled to electrical generators.
The present air-cooling apparatus provides a direct contact chiller with the flexibility for alternative operation as a direct evaporative cooler, and further operable cooperation with ancillary air-treatment structures to pre-cool and control discharge air temperature and humidity. More particularly, this apparatus is coupled to a gas-turbine to provide reduced temperature air without the necessity to provide and operate full-sized vapor-compression system an expensive compressor during peak-load hours, the gas-turbine is connected to an electrical generator.
In an ideal condition, an air precooling system for a gas turbine would provide inlet air to the gas turbine with the maximum increase in air density and control of the inlet air properties, such as temperature and relative humidity. The precooling system would be operable in various modes to systemize the properties of the inlet air while minimize the operating costs and match operating conditions to existing weather and generating load variances. In the case of a gas turbine coupled to an electrical generator, such generators are frequently utilized to supplement normal generating capacity from hydrapower, nuclear power, on-line fossil-fuel combustion or other generating means. A thermal storage apparatus in cooperation with a direct contact chiller, an indirect evaporative cooler and a reheating apparatus can provide chilled air at a reduced temperature and controlled relative humidity at a nominal cost by utilizing off-peak operations to generate a cold mass for reducing coolant temperature for reaction with warm ambient air during any demand period, which is generally a peak on high demand period for gas-turbine generators. This provides a system that can incorporate relatively smaller systems to provide the cold mass, which is usually ice, as it is developed over a long-term such as 12 to 16 hours for reaction with the coolant for a short term during precooling system operation. The economics of the thermal storage system are enhanced by the increased turbine KW output and the increase in turbine efficiency and may be compared to the utility provided incentives to certain customers, primarily for commercial HVAC (heating, ventilating and air-conditioning) operations, to reduce electrical consumption during high-load or peak-load periods. Indicative of these peak load periods are the extremely high mid-afternoon temperatures in the summer months, which in some locales result in "brownouts". In this brownout condition, local utilities resort to purchasing power from other generating plants, if it is available, or operating on reduced voltage outputs or other methods to utilize available power in these high-load periods. Consequently, it is quite evident that utilizing scarce and more expensive power during a high-load period to reduce the turbine air inlet temperature is not economically reasonable. Further, it can be demonstrated that it is possible to continuously utilize a thermal storage system at a controlled rate to reduce turbine inlet air temperature.
An unobvious benefit from the reduction of the air temperature below the dew-point is the recovery of condensate moisture, which is essentially demineralized water, for utilization by ejection into gas turbine combustion zones in the control of nitrogen oxide emissions.